Marine seismic exploration is to investigate physical properties of a formation structure, sediments, and rocks of an underground using a hydrophone which records a seismic wave returning by reflecting or refracting the seismic wave, which is artificially generated from a water surface, from a sub-bottom and may be classified into exploration of resources such as petroleum, natural gas, and gas hydrate, which are blessed in the ocean floor and exploration for engineering for marine construction works, such as a submarine pipeline, burial of a submarine cable, a submarine tunnel, a submarine storage facility, and a bridge. The typical marine seismic exploration acquires data by allowing a exploration vessel to tow a seismic source and a hydrophone or equipping the seismic source and the hydrophone in a ship.
The lower the frequency of the seismic source, the better the transmission of the sedimentary layers but the lower the resolution of data becomes, while the higher the frequency of the seismic source, the more the transmission of the sedimentary layers becomes difficult but the higher the resolution of data becomes. Therefore, for the exploration of resources, a large-capacity air gun and a long streamer having a length of several km are used to understand sedimentary layers having a depth of several km. However, the exploration for engineering is to understand the shallow sedimentary layers in a shallow area and therefore a small air gun of high frequency, a sparker, a seismic source generator such as a boomer, and a single channel streamer or a small multi-channel streamer are used.
In addition, to understand the submarine topography or the shallow sedimentary layers in the exploration of resources or the exploration for engineering, sub-bottom profiling (SBP) is performed using a multi-beam, a side scan sonar, and a chirp. The multi-beam may confirm the submarine topography in a 3D form and the side scan sonar is to image a sea bed. Further, the chirp profiling may understand the shallow sedimentary layers having several tens of meters from the sea bed. The related technology is disclosed in Korean Patent No. 0648917 (Side Scan Sonar Of Ship Bottom Mounted Type Using Multi Array Transducer And Chirp Signal, Registered Date: Nov. 16, 2006). The Related Art Document is illustrated in FIG. 1 as the invention of the same technical field as the present invention and relates to a side scan sonar of a ship bottom mounted type using a multi array transducer and a chirp signal.
Unlike a system using a boomer, a pinger, and the like, having a single frequency seismic source of a short pulse, the chirp transmits a sweep signal of which the frequency characteristics are changed over time, that is, a frequency modulation (FM) pulse. The general chirp signal is transmitted in a sweep form having a pulse length of 20 to 50 ms and a wideband of 1 to 10 kHz and therefore may obtain sub-bottom data transmitted at a depth of about 100 m while having a high resolution of about 10 cm. A main object of the chirp profiling is to differentiate shallow sediments and a succession of strata and therefore the chip is shown in an envelope form having only positive polarity for easiness of analysis. However, the envelope display method disregards polarity and phase information. If the chirp data preserving the phase information are used, a study on acoustic characteristics using chirp data and various studies on chirp profiling for matching with a core sediment structure, classification of sediments using a reflection coefficient, and the like have been conducted. Therefore, a new profiling apparatus for acquiring chirp data recorded in the existing envelope form and chirp data preserving polarity and phase information has been required.